This invention relates to a control arrangement for an internal combustion engine equipped with vacuum operated control devices. More specifically, this invention relates to a control apparatus for applying a vacuum to a control device, locking or retaining the vacuum applied for a specified time or under specified conditions, and then reducing the vacuum in a controlled manner. A preferred embodiment of the invention is well-suited for application to an exhaust gas return (EGR) system of the type which has a diaphragm-operated EGR valve in the EGR tube which connects the exhaust pipe to the air induction pipe.
An exhaust gas return system is known from German Pat. No. 2,822,337. In that system, in order to obtain an effective reduction of the NO.sub.x, the ratio of the total volume of the returned exhaust gas to the volume of the induced air is kept constant independent of the loading on the internal combustion engine by means which, in the exhaust gas return system, control not only internally returned amounts of exhaust gas but also externally returned amounts. A valve, operating in response to the engine vacuum, determines the quantities of atmospheric air returned externally into the by-pass which can be closed by a by-pass valve whose operation is dependent on the exhaust gas pressure existing at any time.
In contrast to the exhaust gas return system described above (which corresponds to an exhaust gas back pressure controlled system) an object of the present invention is to produce a simplified low cost control apparatus for an exhaust gas return system which effectively reduces NO.sub.x emissions.
This objective is attained, in accordance with the present invention, by providing apparatus in the EGR system for locking or retaining the vacuum applied to the EGR value under certain conditions, followed by a reduction of the vacuum in a specified manner.
Particularly in the case of small engines in heavy vehicles, the engine is often operated in or near full load during prescribed emission tests. Since it is precisely in this range that the largest NO.sub.x emissions occur, maintenance of the exhaust gas return in this range is desirable. This occurs, in an arrangement according to the present invention, in a simple manner by locking in the vacuum in the section of the vacuum control tube leading to the exhaust gas return valve. By keeping the exhaust gas return valve open, adequate amounts of exhaust gas return are provided in the upper part of the load range of the engine to reduce NO.sub.x emission to the necessary extent without adversely affecting vehicle driving properties or fuel consumption.
More specifically, an advantageous embodiment of an EGR system which includes apparatus according to the present invention comprises an exhaust pipe, an air induction pipe (induction pipe), a regulating device such as a butterfly valve arranged as a throttle in the induction pipe, a vacuum-operated exhaust gas return valve connected to a vacuum source by a vacuum control tube, a non-return valve located in the vacuum control tube for retaining a vacuum supplied to the EGR valve throughout a range of engine load conditions, and a by-pass valve for relieving the trapped vacuum in a controlled manner. In the preferred embodiment described below, the vacuum control tube is connected to the air induction pipe at a point which is located upstream (on the ambient air pressure side) of the throttle butterfly when the throttle butterfly is closed, but which is located downstream (on the vacuum side) of the throttle butterfly when the throttle butterfly is open. Also, in the preferred embodiment described below, the by-pass valve is a diaphragm-operated device controlled by a pressure tube connected to the air induction pipe at a point which is located downstream of the throttle butterfly when the throttle butterfly is closed, but which is located upstream of the throttle butterfly when the throttle butterfly is open.
An especially preferred compact embodiment of the invention has the non-return valve and the bypass valve constructed in a single multi-part housing. The compact embodiment which includes this single control valve housing has an advantage, when compared with the embodiment comprising individual parts, in that the diaphragm of the by-pass valve can be subjected to differential pressure and a high degree of switching accuracy can be obtained independent of varying absolute pressures (caused, for example, by air filter resistances).
By the addition of a compression spring acting on the diaphragm of the by-pass valve, the exhaust gas return can be interrupted, with proper spring selection, before the engine is fully loaded. By this measure, the thermal loading on the engine can be reduced under full load operating conditions and the performance improved by increasing the supply of fresh air to the cylinders.
Instead of a compression spring, the control valve housing can have a duct (i.e., an orifice), which acts as a throttle, in an intermediate wall of the housing which also contains the non-return valve. The duct makes possible a time dependent opening of the exhaust gas return valve so that the exhaust gas return is maintained only as long as is desirable.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.